Chia-Nan Liu

Behavior of Geogrid-Sand Interface in Direct Shear Mode

The contribution of transverse ribs to the soil-geogrids interaction under pullout mode has been well documented. However, the contribution of transverse ribs to the soil-geogrid interaction under direct shear mode is, at best, unclear. Consequently, this paper presents the results of a comprehensive direct shear testing program aimed at evaluating the contribution of transverse ribs to the interface shear. The direct shear tests involved Ottawa sand and several polyester geogrids with a variety of material tensile strength, percent open area, and aperture pattern. The test results show that the shear strength of sand-geogrid interfaces under direct shear mode is significantly higher than that of sand-geotextile interfaces. Analysis of shear displacement-strength response of the interfaces indicates that, in addition to interface shear components due to sand-rib friction and sand-sand shear at the location of the openings, the transverse ribs provide additional contribution to the overall sand-geogrid interface resistance. Specifically, analysis of the results reveals that the transverse ribs of the geogrid used in this study provide approximately 10% of interface shear resistance. This contribution is positively correlated with the tensile strength and the stiffness of geogrid ribs, but is negatively correlated with the percent open area of the geogrid. A simple model is proposed to quantify the contribution of transverse ribs to the interface shear strength under direct shear mode.

Unsaturated consolidation theory for the prediction of long-term municipal solid waste landfill settlement

The understanding of long-term landfill settlement is important for landfill design and rehabilitation. However, suitable models that can consider both the mechanical and bio-decomposition mechanisms in predicting the long-term landfill settlement are generally not available. In this paper, a model based on unsaturated consolidation theory and considering the biodegradation process is introduced to simulate the landfill settlement behaviour. The details of problem formulations and the derivation of the solution for the formulated differential equation of gas pressure are presented. A step-by-step analytical procedure employing this approach for estimating settlement is proposed. The proposed model can generally model the typical features of short-term and long-term behaviour. The proposed model also yields results that are comparable with the field measurements.

Modeling municipal solid waste landfill settlement

A good prediction of solid waste landfill settlement is important for landfill design and rehabilitation. A one-dimensional model which accounts for mechanical settlement and biodegradation processes is developed to simulate the settlement behavior of municipal solid waste landfill. The derivation of analytical solutions for specific conditions is introduced. The numerical approach, capable of coping with more general conditions, is also presented to estimate the spatial and temporal distribution of landfill settlement. The proposed model can simulate typical features of short- and long-term landfill settlement behaviors. With proper selection of parameter values, field measurements are well simulated by this model. The effects of some design parameters on the settlement behavior of municipal solid waste landfills are also examined with the help of this model.

Estimating spatial correlation structures based on CPT data

Spatial correlation structures are usually used while modelling inherent uncertainty associated with soil properties. To retrieve this information a sufficient number of measurements and equal intervals between measurements or samples are always required. However, in practice, it is not usual to conduct site investigation in that way subject to a limited budget. Therefore, it is of interest to understand the possible bias created by using samples with unequal intervals. This research focuses on estimating the vertical spatial correlation structures of cone penetration tests (CPT) data conducted at a liquefaction site in Taiwan. The appropriate fitting range used in calibrating the correlation model is also discussed. The results show that cone tip resistance has larger correlation distances than that of sleeve friction. The estimated correlation distances of the CPT profiles are similar while using an equal sampling interval within a certain fitting range (denoted as the critical interval of measurements in this research). The results indicate that the effect of using different curve-fitting ranges is relatively insignificant for correlation distances of sleeve friction, whereas the correlation distances of cone tip resistance are more sensitive to different curve-fitting ranges. Moreover, the mean values of correlation distances are more constant for the equal sampling interval cases than for the unequal interval cases.

Landslide Hazard Mapping Using Monte Carlo Simulation- a Case Study in Taiwan

Taiwan’s slope disasters resulted from heavy rains happen frequently due to its geographic, weather and topographic conditions. Therefore, the slope disasters in Taiwan cannot be neglected. With the development of GIS, the infinite slope analysis method which uses deterministic concept will confront two major problems when it is used in evaluating landslide susceptibility: first, the various engineering parametrics can’t be determined easily, second, the mutual independent between grids will cause the unreal conditions such as the low landslide potential grids surrounded by high ones. This study chooses route Tou-71—from Puli to Ren-Ai as the main research area and typhoon Toraji as the rainfall event of analysis. The study also use the wide-range slope stability analysis software—TRIGRS, and Monte Carlo simulation which considers the uncertainty in parametrics to replace traditional single factor of safety with probability ones. The landslide susceptibility of study area is described by potentially failure probability.

Analysis of a reinforced slope failure induced by the Chi-Chi (Taiwan) earthquake

Abstract A reinforced soil slope located at the entrance road of National Chi‐Nan University failed during the Chi‐Chi (Taiwan) Earthquake. The theme of this paper is to provide an insight into the design, construction, and failure of this reinforced slope. The location, geometry, and configuration of the slope are described. A detailed field observation of slope failure is made, in order to collect information about materials, construction, and failure mode. The soil and reinforcement materials retrieved from field are tested for strength properties. Slope stability analyses including pseudo‐static and stress deformation analyses are performed. Pseudo‐static analysis reveals that the failure is likely induced by a pseudo‐static coefficient between 0.4 and 0.5. The stress deformation analysis reveals that shear strain is initiated at the top and bottom connections between reinforced zone and natural soil. The critical failure surfaces estimated from field observation, pseudo‐static analysis, and stress deformation analysis compare reasonably well.

Backfill Stress and Strain Information within a Centrifuge Geosynthetic-Reinforced Slope Model under Working Stress and Large Soil Strain Conditions

Numerical methods combined with a centrifuge test are used to investigate the mobilization of backfill stress and strain within a geosynthetic-reinforced soil (GRS) slope under working stress and large soil strain conditions. System stability indicated by the factor of safety (FS) of the GRS slope is calculated using limit equilibrium analysis. The stress and strain information under various soil stress states is evaluated using a finite element model with a soil constitutive model capable of modeling soil softening behavior. The numerical models are verified by data from a centrifuge GRS slope model. Numerical results indicate that soil stress mobilization can be described with soil stress level S, which is defined as the ratio of current stress status to peak failure criteria. As loading increases, backfill stresses develop and propagate along the potential failure surface. Mobilization of soil stress was non-uniform along the failure surface. Immediately after the stress level reaches peak (S=1), strength softening initiates at the top and toe of the slope at approximately FS=1.2. The slope settlement rate and reinforcement tensile load significantly increase when soil softening begins. The softening occurs randomly and irregularly along the failure surface and the formation of soil softening band completes at approximately FS=1.1. The failure surface corresponds to the locus of intense soil strains and the maximum tensile loads at each reinforcement layer.

Working-strain based design of landfill final cover systems

Abstract An approach to estimating the strain within geosynthetics placed on a landfill cover slope is emphasized. This approach adopts the analytical method for evaluating the geosynthetic tension within a geosynthetic/soil multi‐layered slope system. A review of the assumptions, solutions, and capabilities of this analytical method is summarized. The proposed approach is applied in stability analysis of an example landfill slope to evaluate the geosynthetic strains for different lengths of cover soil placement. The corresponding factors of safety are calculated using conventional slope stability analysis method. The results show that the relationship between the factor of safety and the slope length is convergent, while the relationship between geosynthetic strain and slope length is divergent. This analysis reveals that strainbased design is more rational.